Particle manipulation by using ultrasonic standing waves (USW) in microfluidic devices may commonly be used in bioseparations, characterization and analysis of micron-sized species.
A great number of publications highlighted the utility of USW by showing how particles, cells and bacteria can be affected by acoustic radiation forces.
Primary radiation forces may allow species to levitate at the nodal plane in a resonator. Inhomogeneity of the transversal energy distribution and non-idealities in geometry of the resonator may lead particles to agglutinate at the nodal plane and secondary Bjerknes forces may keep aggregates consolidated and stable.
Aggregation of cancer cells for instance can be generated in cylindrical resonators in order to study physiological cell interactions.
Species of a few micrometers or more may be trapped by using USW and can be focused at the nodal plane of a standing wave and kept closely immobile.
When species are smaller than a few micrometers, Brownian agitation may not be negligible and acoustic and thermodynamic forces may enter in competition for establishing a steady state after a relaxation time.
The acoustic streaming (corresponding to Rayleigh streaming in the present disclosure) may become observable when a suspension is composed of sub-micronic particles. In this case, the displacements of fluid may drag the species over the whole volume of the channel modifying thereby the thermodynamic equilibrium.
Several works have been devoted to the observation and description of the acoustic streaming in particle and cell manipulation and it was observed that acoustic streaming may generate clumps and recirculation of particles at nodal planes.
Acoustic streaming may be present when acoustic radiation pressure is generated by USW but primary radiation forces may veil it if size of objects is sufficient.
THE acoustic streaming can be visualized by using Particle Image Velocimetry (PIV) and it was shown that the flow velocity generated by the streaming may be strong enough for making manipulation of sub-micronic objects relatively difficult.
A need therefore exists to obtain a method allowing controlling the impact of the acoustic streaming, especially on relatively small objects.
A need also exists to obtain a method allowing reducing the impact of the acoustic streaming on objects while keeping acoustic forces strong enough for allowing a satisfying focusing of said objects.
The present invention aims to meet one or more of the aforementioned needs.